This invention relates to electrophotography, and more particularly, to an improved overcoating layer for an electrophotographic imaging member.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photo-generating holes and injecting the photogenerated holes into the charge transport layer.
Other composite imaging members have been developed having numerous layers which are highly flexible and exhibit predictable electrical characteristics within narrow operating limits to provide excellent images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an anti-curl back coating and an overcoating layer.
Imaging members are generally exposed to repetitive electrophotographic cycling which subjects exposed layers of imaging devices to abrasion, chemical attack, heat and multiple exposure to light. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the exposed layers. For example, repetitive cycling has adverse effects on exposed portions of the imaging member. Attempts have been made to overcome these problems. However, the solution of one problem often leads to additional problems.
In electrophotographic imaging devices, the charge transport layer may comprise a high loading of a charge transport compound dispersed in an appropriate binder. The charge transport compound may be present in an amount greater than about 35% based on weight of the binder. For example, the charge transport layer may comprise 50% of a charge transport compound in about 50% binder. A high loading of charge transport compound appears to drive the chemical potential of the charge transport layer to a point near the metastable state, which is a condition that induces crystallization, leaching and stress cracking when placed in contact with a chemically interactive solvent or ink. Photo-receptor functionality may be completely destroyed when a charge transport layer having a high loading of a charge transport molecule is contacted with liquid ink. It is thus desirable to eliminate charge transport molecule crystallization, leaching and solvent-stress charge transport layer cracking.
Another problem in multilayered belt imaging systems includes cracking in one or more critical imaging layers during belt cycling over small diameter rollers. Cracks developed in the charge transport layer during cycling are a frequent phenomenon and most problematic because they can manifest themselves as print-out defects which adversely affect copy quality. Charge transport layer cracking has a serious impact on the versatility of a photoreceptor and reduces its practical value for automatic electrophotographic copiers, duplicators and printers.
Another problem encountered with electrophotographic imaging members is corona species induced deletion in print due to degradation of the charge transport molecules by chemical reaction with corona species. During electrophotographic charging, corona species are generated. Corona species include, for example ozone, nitrogen oxides, acids and the like.
A number of overcoating layers have been proposed for various purposes. U.S. Pat. No. 4,784,928 to Kan et al. discloses a reusable electrophotographic element comprising first and second charge transport layers. The second charge transport layer has irregularly shaped fluorotelomer particles, an electrically nonconductive substance, dispersed in a binder resin. The second charge transport layer allows for toner to be uniformly transferred to a contiguous receiver element with minimal image defects.
U.S. Pat. No. 4,260,671 to Merrill discloses various polycarbonate overcoats which provide an increased resistance to solvents and abrasions.
U.S. Pat. No. 4,390,609 to Wiedemann discloses a protective transparent cover layer made of an abrasion-resistant binder composed of polyurethane resin and a hydroxyl group containing polyester or polyether, and a polyisocyanate.
There continues to be a need for improved overcoatings for electrophotographic imaging members, which overcoatings will provide better protection for the charge transport layer from adverse mechanical- and chemical-induced effects.